For purpose of proper understanding of the background of this invention and to avoid the repetition of details herein not primarily relating to the advance in the art contributed by this invention, the preceding application is incorporated herein by reference in its entirety, and will only be reproduced herein to the extent that the relationship between the prior application and this invention is understood. Thus, various circuits for load and alarm indication by means of slip analysis is shown and described adequately in said U.S. patent, and is not repeated herein. However, circuit configurations are shown herein showing how to compensate for variations in temperature and voltage to improve the accuracy of load indications derived by slip analysis. The said patent together with patent references of record therein constitutes the best known prior art.
While the former patent provides an output signal indicating motor load as a function of armature slip and works well with 5 to 10% accuracy requirements, it is limited where improved preciseness and accuracy is desired. Particularly with large horsepower industrial motors, there is a significant change of armature slip with motor operating temperatures. Thus, if accuracy of the readings under these operating characteristics is to be kept within 1% to 5%, the previous systems are not acceptable. Furthermore, changes of applied motor voltage also causes variations of armature slip that may prevent readings from being accurate under all conditions within the desired improved accuracy limits.
Another problem with motor load metering and monitoring systems and particularly those of a type not operable on an armature slip analysis, is the need to retrofit motors in the field with access to complex electric wiring, armature current, etc. There is no really low cost simplified retrofit or even factory installed monitoring system available that produces high accuracy loading signals or displays.
Also, armature slip characteristics have not previously been understood properly so that they could be processed for highly precise and accurate load indications. For example, the effect of motor temperature and motor voltage upon slip characteristics has not been taken into account in previous load monitoring systems dependent upon slip analysis.
In order to make temperature and/or voltage corrections to improve accuracies in prior art load monitoring systems such as those operating from armature current characteristics, or from interpretation of mechanical distortions such as shaft torque, difficulties are imposed because of incompatibility of sensors, signals and relationships between motor, load, temperature and voltage characteristics. Thus, very complex equipment including a plurality of sensors at different locations providing mounting, connecting and interfacing problems would be necessary for correcting load signals to produce more precise and accurate displays.